So. This exists.
There's a small window of vulnerability during the beginning of the otherwise invulnerable first ledgesnap. For a lot of characters with one-dimensional recoveries, this proves to be problematic with characters who have reliably long-lasting hitboxes and spikes. Especially for Robin, who has no hitboxes to combat anyone on the way up, making him a big target. Ordinarily, someone could simply wait at the ledge and get ready to spike when we recover.
Fortunately, we're one of the few with such a recovery that can readily combat a prepared spiker, thanks to Arcfire and Arcthunder. Arcfire ignites on contact with anything regardless if it was shielded or not and keeps its hitboxes out, preventing anyone from intercepting Robin without committing to a jump, and by the time Arcfire's been jumped over, Robin should have recovered and escaped the vulnerable window. Similarly, Arcthunder goes into its multihit effects when shielded, leaving the opponent in shieldstun or having to roll to escape, at which point, again, we would have recovered; spotdodging I would think would let us escape the window as well.
It's going to be imperative to have an Arcthunder stored up whenever we're knocked offstage, in case our Arcfire tome is gone. Master your defenses when recovering. And it will also be important to know when and where we can shoot Arcfire so that it lands on the stage, on the corner, or on the ledge, depending on where the interceptor's at.
As far as Robin being the interceptor to ledge-seeking foes, well, we have a frame 8 spike. That we can use from the stage. Safe to say we should master the timing down. It would make the Lucario, Sheik, and Megaman matchups a hell of a lot easier if we do.
If for some crazy reason we have run out of Elwind, we have the alternate method of grabbing the ledge, dropping and double jumping when the foe is about to grab the ledge, and Levin Dair spike during the window while we take advantage of our ledge invincibility that carried over in order to prevent being hit by the UpB or other attacks. But since Dair is Frame 12, it should only really be used on those with startup on their UpBs. Falcon, Sheik, Zelda, Lucario, Villager, Falco, Duck Hunt, UpBs of that sort.
Did I get the general gist down? Or are there things I missed from that brief analysis?